Heads-up displays, also sometimes known as near-to-eye displays, allow a user to view a scene in front of them while relevant information is overlaid on the scene, so that the user looking through the heads-up display simultaneously sees both the scene and the relevant information. For example, a pilot looking through a heads-up display while landing an airplane simultaneously sees the airport ahead (the scene) through the heads-up display while the heads-up display projects information such as speed, heading and altitude (the relevant information) that the pilot needs to land the plane.
In some instances it can be useful to couple a heads-up display with a camera that can capture the scene being viewed by the user. A potential difficulty with coupling the heads-up display with a camera is that of ensuring that the scene captured by the camera is the same as the scene viewed by the user, and that the captured image is correctly registered with the viewed image.